In machine tool spindles, a clamping device (tool chuck) is frequently provided inside the hollow machine tool spindle. This clamping device comprises a connecting rod which extends coaxially with the spindle axis and the front end of which in the region of the tool holding fixture is provided with a collet chuck engaging the tool. Through the rear end of the connecting rod extends a hydraulically operating, so-called release unit which serves to operate the clamping device. So that a cooling lubricant can be supplied to the tool, the connecting rod is hollow and its end protruding from the release unit is connected to the central throughflow channel of the hollow shaft. The rotating union thus allows supply of the cooling lubricant from the stationary housing to the rotating connecting rod. On changing the tool, i.e. when the machine tool spindle is at a standstill, instead of cooling lubricant, air for blowing out the tool holding fixture can be passed through the rotating union and the connecting rod. While the machine tool spindle is at a standstill, hydraulic oil can be supplied at high pressure to the release unit via the second inlet channel, the slide ring packing assembly and the parallel second throughflow channel, whereby the release unit displaces the connecting rod in the machine tool spindle and the connecting rod releases the workpiece.
Rotating unions for two different fluids are well-known in the most varied designs and for the most varied purposes. A rotating union of the kind described hereinbefore, such as might be used for the purpose described in combination with machine tool spindles, is shown and described for example in catalogue 869 D "DEUBLIN ROTATING UNIONS" of the firm Deublin-Vertriebs-GmbH, D-6238 Hofheim-Wallau, page 36. This rotating union has various drawbacks when used for machine tool spindles as described. It is suitable for a maximum speed of only 1,500 r.p.m. But for machine tool spindles nowadays, maximum speeds of 30,000 r.p.m. or more are required. If wet and dry machining alternate with each other, then blowing the cooling lubricant out of the rotating union and the connecting rod every time the tool is changed is extremely troublesome. For the fact is that the workpieces or measuring tools, which are to be kept dry, are wetted with cooling lubricant. Moreover, the operator is annoyed by the liquid mist. It must be taken into consideration here that even in case of continuous dry machining in which no cooling lubricant is to be supplied to the tool, there must nevertheless be cooling lubricant in the rotating union so that the central slide ring is lubricated and frictional heat is conducted away. If there is no cooling lubricant at the slide ring packing, then the slide ring packing is soon destroyed, particularly at higher spindle speeds. Even in case of dry machining, therefore, cooling lubricant must be supplied to the rotating union again after every tool change after blowing out the tool holding fixture. So that this cooling lubricant does not pass to the tool during dry machining, in the connecting rod or hollow shaft is mounted a spring-loaded shut-off valve which shuts off the throughflow channel in the hollow shaft or hollow connecting rod during dry machining, in which the cooling lubricant is supplied to the rotating union at low pressure. This non-return valve however gives rise to further problems, for it also prevents, among other things, throughflow of the cooling lubricant during wet machining. Furthermore, the central slide ring is only cooled inadequately if a non-return valve is used, because no forced circulation of the cooling lubricant is provided. For this reason too, the known rotating union is not suitable for higher spindle speeds. Also, only the central slide ring separates the cooling lubricant flowing centrally through it, from the hydraulic oil surrounding it. So that the two fluids do not become mixed with each other, a relatively strong compression spring must act as a basic load on the inner slide ring support. This higher basic load then leads to unnecessarily high pressure of the central slide ring on the common slide ring during dry machining too, which is connected with corresponding generation of heat and wear. Lastly, in case of dry machining, on account of the necessary cooling and lubrication of the central slide ring by the cooling lubricant, it is not possible to operate with a supply of compressed air. But such supply of compressed air is sometimes desirable to cool the tool and the workpiece and remove cuttings.
It is therefore the object of the invention to provide a rotating union for two different fluids, in particular for machine tool spindles, of the kind mentioned hereinbefore, which is particularly suitable for very high speeds while avoiding the above drawbacks, in which throughflow through the central throughflow channel can take place unhindered, and which allows dry machining even at high speeds, if required with the supply of compressed air.